CHAP. H.] 



LIGHTHOUSES. 



221 



light, that seafarers instantly asked them to return to the primitive 

 method of the middle ages." (Lighthouses, by Leon Kenard.) 



Teulere, an engineer of the last century, substituted mirrors of 

 parabolic form in place of spherical ones. The light from a lamp 

 placed at the focus of the mirror of this kind, is sent out in a cylin- 

 drical beam, the intensity of which does not, therefore, diminish with 

 distance. The only lessening of light is produced by the absorption 

 by the atmosphere or fogs. For the ordinary lamps, the same inventor 

 also substituted lamps with the 

 double current of air invented by 

 Argand; later on, Carcel's lamps 

 in which the oil is brought to the 

 burner in a continuous manner by 

 clockwork, again increased the light 

 and steadiness of the burners. 

 Teulere was the first to make 

 his mirrors revolve round a lamp, 

 the burner remaining in the axis 

 of rotation, so that the light was 

 successively projected to every part 

 of the horizon, and then eclipsed. 

 He is also the inventor of the re- 

 volving ligLt. A lighthouse of 

 this kind was erected at Dieppe 

 by Borda, in 1784, and another in 

 the tower of Cordouan, six years 

 later, in 1790. 



The catoptric apparatus is gene- 

 rally composed of groups of parabolic mirrors, each having a lamp at 

 its focus. The whole is moved by clockwork. One is represented in 

 Figure 154. It comprises three systems of reflectors, themselves 

 grouped in three, so that a complete rotation gives to each parb of 

 the horizon three illuminations and three eclipses. By varying the 

 velocity of the movement, eclipses may be obtained more or less 

 rapidly, and thus the lighthouses established on different parts of 

 the coast, may be distinguished from each other. 



The range of the parabolic mirrors is considerable. Experiments 

 due to Biot and Arago prove that a mirror O m - 81 aperture gives a 



Fiu. 154. Catoptric light. 



